Rubber mats having projections at the lower side thereof are utilized in track construction and function to support the ballast bedding as well as the ties and rails placed thereupon with respect to the ground and with respect to tunnel walls and trough walls in order to attenuate the transmitted sound.
German patent publication 3,425,647 discloses a mat of the kind described above. This mat has a high-strength fabric at its upper side which is configured to have two layers. The projections at the lower side of the mat can be configured as steps or as truncated-conical projections or as cylindrical nubs. The steps have a trapezoidal shape when viewed in cross section.
The production of mats of this kind of elastic base material is carried out stepwise in a heatable plate molding press. To achieve the projections formed at the lower side, it is necessary to provide a plurality of mold cavities spaced closely to each other in the plate form press. These mold cavities have the negative shape of the projections. In each heating step, approximately 33,000 mold cavities are arranged, for example over a 15 m.sup.2 press surface.
When forming the projections in the mold cavities, it is necessary to provide each mold cavity with venting channels so that air and other gas, which is trapped as the green rubber is pressed in, can be conducted away. If venting of the mold cavities is not effective, then the projections are poorly formed because the plastic rubber cannot completely fill out the mold cavities.
These venting channels cause the so-called flashing or excess rubber which is rubber pressed into the venting channels during the flow phase. The excess rubber must then be removed with difficulty in a later work step.
A method for making flat rubber hollow bodies, which can be inflated, is disclosed in German patent publication 4,040,591. In this method, a press mold is used wherein the mold inner wall of one or both form halves is provided with a fabric extending beyond the mold space. With this arrangement, it is intended to improve the removal of the air and gas during the vulcanization step. This lining of a relatively large mold for a unitary molded body cannot be realized for a plurality of mold cavities close to each other. In addition, the product which is formed is provided with an impression of the fabric, which lines the mold contour, and this is not always wanted. Furthermore, venting channels leading to the outside for conducting away the air are necessary.